What is the greatest common factor of $28$ and $40$ ? Another way to say this is: $\operatorname{gcf}(28, 40) = {?}$
Answer: The greatest common factor is the largest number that is a factor of both $28$ and $40$ The factors of $28$ are $1$ $2$ $4$ $7$ $14$ , and $28$ The factors of $40$ are $1$ $2$ $4$ $5$ $8$ $10$ $20$ , and $40$ Thus, the greatest common factor of $28$ and $40$ is $4$. $\operatorname{gcf}(28, 40) = 4$